Curve modifier does not work when you merge another curve to the original curve
The Curve Modifier only reads the first spline segment of the curve object. 

Either join all segments into one continuous spline, or use separate objects for each stretch of track.

There are also a myriad of issues with your scene, one of each is unnapplied scale on the mesh. See my answer for common issues with the Curve Modifier

Besides that, the root of the problem seems to be extraneous geometry in your curves. Both of them seem to have loose vertex that unexpectedly make them multi spline curves causing unexpected behavior.

Erasing loose geometry and applying scales should solve your issues. TO easily erase all loose geometry, select one vertex of your main spline, then press Ctrl + L to select all linked vertex. 

Afterwards press Ctrl + I to invert selection, effectively selecting all vertex that don't belong to the main spline, then you can erase them.